Related Clues:

• Give a heads-up to
• Raise red flags
• Notify of potential danger
• Give a heads-up
• Issue a caveat
• Give an alert
• Sound the alarm
• Notify in advance
• Honk at, perhaps
• Shoot off a flare, e.g.
• Notify of peril
• Alert to danger
• Serve notice
• Premonish
• Shout "Fore!" to